RFR: 8061748: Remove check_ct_logs_at_safepoint()

[Kim Barrett]

I would like reviews of this patch to remove the unused function
G1CollectedHeap::check_ct_logs_at_safepoint(). It appears to have
never had any callers since it was added as part of the initial G1
integration:
https://bugs.openjdk.java.net/browse/JDK-6711316
http://hg.openjdk.java.net/jdk9/hs-gc/hotspot/rev/37f87013dfd8

The purpose of this function is to perform a consistency check between
the card table and the dirty card logs. If something along those lines
is needed for some future debugging task, it's easy enough to write a
bespoke test when that time comes.

Removal of this function also renders the helper class
ClearLoggedCardTableEntryClosure unused, allowing it to be removed as
well.

The underlying purpose of the removal is to eliminate one of the two
callers of CardTableModRefBS::non_clean_card_iterate_serial(), with
the intent of later eliminating the other call and allow that function
to be removed, thereby eliminating one of several different
implementations of iteration over card tables.

Webrev:
http://cr.openjdk.java.net/~kbarrett/8061748/webrev/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8061748

Testing:
local jtreg for hotspot/test/{gc,runtime} hotspot/test/closed{gc,runtime}
jprt -testset hotspot